*Riot Retro Gaming* - th-cam.com/users/RiotRetroGaming *RRG interview of me* - th-cam.com/video/DFc3SHTlUDo/w-d-xo.html *Diagnostics Carts from Shareware Plus (Stockport)* - www.ebay.co.uk/usr/tim685?_trksid=p2047675.l2559 *Jan Beta (Voltage regulator replacement with DC to DC)* - th-cam.com/video/I8OPUC47CIQ/w-d-xo.html *If you would like to support the channel via Patreon (more support = more videos)* - www.patreon.com/GadgetUK164
GadgetUK164 - Retro Gaming Repairs & Mods Another quality video. Watching now. Still humbled and cannot express my thanks enough. This would of cost me £££ - I'm celebrating by 'eaching' (eating and watching) with some garlic bread and spag-bol 😛😎
Those copper heatsinks are expensive but really good. I use 4 on my VIC and 3 on the PLA typically for my personal breadbins.. And yes, I just started doing repairs back in February starting with my own personal C64 which needed a character rom and 1 bad RAM chip.. I have all the tools all you guys have and have learned so much in such short time so thanks for sharing the knowledge. Its a great hobby, but can get expensive and addictive... Not saying I have a problem, but why do I need 5 C64s! (I just made 4 BWACK c64 savers).
Nice work and thanks for the mention. :) I had very similar confusing results with a broken 7708 and the dead test cartridge a while ago. It seemed to randomly flash differently on each power up. I replaced two RAM chips and broke a trace before I realized it was the multiplexer. ;)
I had a similar quest with a 7707 chip in my 1541. The way I finally proved that that chip was faulty was removing it from the board and testing it in my TL866 EPROM programmer which has a very handy test option for a range of 74-series chips. Since the 7707 generic replacement is a 7406 I tested it as that and it told me one of the logic units had failed. The TL866 also tests SDRAMs so quite useful. Drive worked again fine after replacing that chip with a 7406.
On mine with a black screen, U 10, 12, 22, 24 all 4 are very hot within 30 seconds. The other 4 are cool. I will be replacing all 8 Hitachi RAM chips and both 257's and hopefully this solves the problem. I will test further with my oscilloscope.
In this video I used a "Logic Probe" - which indicates a high / low / pulsing / high impedance signal, and I also use an Oscilloscope to measure clocks.
Good job on working your way to the solution by testing and solid reasoning skills. One thing I thought of while watching is that if one has a eprom programmer like the TL866 you can test 74xxx series logic chips with it. I have not tried this yet but was about to this weekend on a difficult TRS-80 Model 100. In a case like you had where the symptoms indicated a bad 74xxx part and you don't have one on hand it might be worth hooking up the programmer to test the suspect part.
Thanks! =D Yes, I wish I had an EPROM programmer that could test TTL (I've got 5 different EPROM programmers and not one of them tests TTL lol). I was going to create my own TTL tester using a PIC, I've not got around to it yet but I will do at some point! In the meantime I might buy a TL866!
I have a fully socketed board build coming up =D Regards testing the SID, you really need to familarise yourself with how a 6581 and 8580 sound with a number of different games. Then always use those games to test. It's easy to spot when there's a missing channel or bad filter etc. But sometimes problems with ADSR stuff can be subtle.
Thanks for a great video. It helped a lot with me troubleshooting my black screening C64. Not sure what it was in the end. Measured the fuse, voltages of the voltage regulators which showed very good. Tried removing the SID. Still just black. Then I measured the input voltage on the 74-series chips and it showed only 2.2V... was a bit confused and went back and forth between the regulator and the input and after scratching my head for a moment the input was suddenly showing 5V so I hooked it up and it was running fine. So not really solved as it might return for all I know but this video helped very much in me daring to open up and check around at least =)
I might be misremembering but I recall going back and forth between the output pin of the 7805 voltage regulator showing 4.99V and the input pin on several chips showing 2.2V. Not sure if there could be a failing component somewhere bringing down the voltage or a cracked solder joint somewhere that got better when it heated up. Anyway the video helped a lot and none of the caps looked damaged so not going to repair further that which is no longer broken or it might never work again =)
There are 2 5V rails! The regulator on the C64 motherboard provides 5v ONLY for the VIC and chips next to it. The PSU provides a second 5v rail which feeds most of the other chips on the board. So if you see a low 5v elsewhere on the board (other than the VIC or associated chips in that area), then either your PSU has a low 5v level, or more often than not its the power switch or power socket that is dirty. But you could also have a short somewhere, that would pull the 5v down, but shorts of that magnitude don't generally fix themselves.
Yeah i watched that interview of you on Riots channel Gadget..nice to finally see your face..i suggested they interview Jay from Jays vintage Junk sometime too!!!
For schematics, you should look at the "Assy no", not the "Artwork no". If the schematic is for the right assy, they are normally correct, even is the artwork number is different. So in your case your have an assy 250407 and I'm sure the schematic you have matches with that. Even for breadbins, there exist several assy's and their schematics differ (clock circuit and RAM mainly).
Yes, I had forgotten that the clock circuit can vary between boards too - that's a very good point! But in general, assuming you dont have clock problems, you can use pretty much any schematics to check connectivity. The IC numbering can confuse you too I guess if you get schematics for the wrong board.
I agree with Retro Marky. The best video I've ever seen on a C64 repair explaining the various test procedures when working on a dead board. I've found with the MOS 74LS IC's they are now prone to failure and worth checking / replacing after initial tests (to hopefully help cut repair time). I've also scoped the output pins of some suspect / faulty MOS Multiplexer IC's finding they were still swinging high and low but not sufficent enough to change logic levels. As for Diagnostic cartridge with RAM, even the Micro Sleuth 64 (with seperate LED Userport board ) or the Diagnose 64 (with onboard diag LED's) don't have any RAM. Could the "RAM" have actually been just 74LS logic IC's or ... there were a few programmable battery backed RAM cartriges like the Trigsoft UPC1 could this be what you were thinking of?
Thanks =D Very much appreciated =D I think the cart I used to use at Eteson Electronincs was a homebrew cart - it had that appearance tbh, and I've searched high and low and not found anything that looks like it (either on the board with RAM, on regards how it displayed on screen).
mate this video is SUPER helpful ; i have a dead c64 with white flashing on my diag cart. the mem has been socketed ; my suspicion is the 7708's over the RAM as well but your troubleshooting has me really thinking its them. will socket em up and check. thanks mate
Thanks =D Just be aware that other things can cause black screen (like the PLA, CPU, CIAs, SID etc). But if its not the PLA or RAM then the 7708s are very possible.
yep! i've had the PLA tested in another machine and it works fine. the SID is non existent and the CIAs have been socketed and removed. am told the RAM was replaced (but unsure) so will try some 257s and see how we go.
Fantastic video Mr.G. The best video I've ever seen on a C64 repair. So glad you went into the logic probe. Could you consider making a video on using a scope to diagnose C64 problems? I think a lot of people including myself would learn a lot from that and you're the man to do it ;)
Excellent video as always! Lots of red herring opportunites indeed, but you found a solid footing quick by being methodical. Interesting failure mode on those 7708:s
There is no need to apologise ;-) Its just your content is so good I want to watch but sometimes when there is too much movement in front of camera the DMS kicks in. I used to think it was just me but its quite common - well.blogs.nytimes.com/2015/11/14/feeling-woozy-it-may-be-cyber-sickness/ for instance I don't get the problem with say Jay's stuff for instance, he had the camera just that little further back.
What a pain in the arse. I do enjoy the fault findings but it had me stumped. Glad you got it repaired. Another C64 resurrected...... Great vid Chris 😁😁😁 Kim....
I am sure they fail a fair bit, but I've got loads of C64s with those chips on that work fine atm! Ultimately I am sure they will die before other makes perhaps though.
Thanks for this awesome video! I'm working on that exact board. Can't really figure out whats happening. 8500 gets very hot, no phi2 from it. Kernal and CHR is being addressed but not Basic. D4 is stuck, other than that, there is data on the bus. Removed CIA2 and SID since they were socketed. Any further tips on what I should check next? I have an oscilloscope and another C64, but It's a shortboard without any sockets on chips that are compatible.
If there's no PHI2 from the CPU, I would suspect the CPU tbh! I could be wrong, but I think the VIC2 provides the clock to the CPU, and the PHI2 should be created by the CPU.
I have used the Recom 5V regulators in many places, but I wouldn't recommend them for the ones on the C64 as they both drive analogue circuitry and don't use that much power, so would be better off with the original linear regulators.
Yep, that seems to be what Jan found. Spike on the 12v side after power turned off (probably due to capacitors). It did make me wonder if a similar spike was created on the 5v side too though.
Investigation is ongoing. I replaced the 12V Recom one with a regular 7812 for now. I'm using the C64 a lot and haven't found any more issues yet. At least nothing you can tell from the outside except the whole machine is obviously running cooler. I'll watch for spikes on the 5V rail when I open it up again.
The power supply itself provides the 5v for the other TTL chips! The regulator only powers the VIC chip and the surrounding chips in the shielded area. Check the 5v on the power connector on the PSU, and check the fuse in the PSU hasn't blown.
I had saw a video, maybe yours, where you measured 5VDC at the 5VDC regulator and then measured 5VDC at the TTL chips.. I assumed that voltage for the TTL's was supplied from that regulator! My mistake. Ah Ha! Just checked..Less than 2 volts coming off of C91 (and the TTL chips). Maybe something is loading it down(?). Good power supply tested in a working unit. Thanks! EDIT------------------------------------------- Just watched the video again and you explained the separate 5 volt sources in the text. I should pay more attention. 3:42 4:10
Provided you don't use too much heat they are OK to work on! Sinclair Spectrums are much more difficult. If you have little desoldering experience its best to get a cheap desoldering station as it makes chip removal much easier. Something like the ZD-915.
I find that if I cut the chips out when I replace them it makes desoldering easier and you don’t spend forever heating the area trying to get chips out.
I wonder why you didn't use your 1541U2 for diagnosing the RAM? It needs just PLA, VIC and somewhat functioning RAM to display a picture on the screen. Inserting it and activating the menu is very usefull for RAM diagnostics: If one line in every character is broken, you have a broken RAM chip. If you see total garbage on the screen, something else is going on.
That's a nice tip! But the main reason I never connect the 1541U2 up whilst fault finding is the risk that a fault could damage the 1541U2. In this case it wasn't likely, but there are some black screen faults where say a data or address bus is stuck high, which can kill other things connected to that bus. I've seen 64C's with a dead CPU, and 2 x dead CIAs - all with the same data bus connection killed. If I connected up a 1541U2 to that, it could damage it! The only time I will use the 1541U2 is when I know its working lol.
Yes, I think some pre-checks are usefull, but I've succesfully used the 1541U2 in 2 black screen repairs until now: In the first machine, all ROMs were socketed, I removed all of them (even char ROM not needed to 1541U2). After removing all of them, 1541U2 menu worked fine and using the altenate KERNAL function, I was able to get empty blue screen. So RAM was fine, no need to worry about that, and one of the ROMs had to be the cause. It turned out that the BASIC ROM was spoilng the bus, replaced it. C64 working fine. For the other machine, the 1541U2 menu did show immedeately that one of the RAM chips was defect. Replaced it, machine was working fine for 2 weeks, then broke again :( I'm sure the RAM is correct, I have socketed all RAM chips and tried many things there, but as the 1541U2 menu displays perfectly the RAM is unlikely the cause now. I'm considering desoldering the CPU, but it'll have to wait until I'm in the mood again to look at it.
What's your opinion about the heat in the VIC chip having a large mobo like the ones of the breadbin?. Would you recomend to keep the plate over that chip there to dissipate the heat instead of using a heatsink?. Or thinking about mods, wouldn't it be better to add a fan over that cover instead of using a heatsink with a fan on the VIC? Or in the case that both solutions could work more or less the same, do you recomend using a fan on the VIC?.
I think it would be better to probably just have a dedicated heatsink and remove the top tbh - but... That top joins the side piece etc and acts as a larger heatsink. So I guess it would be worth testing in both scenarios to see which way the VIC is actually cooler. Personally, after seeing how hot my VIC chip gets I prefer a fan tbh. On the C64C it doesn't get so hot though.
I wonder if Commodore repaired and repackaged systems, could explain the mismatched chips . Also what do you think could cause a cartridge port from not working besides power or ground issues, I'v replaced my PLA chip with a OTP rom chip after a power supply took it out and the system boots up and loads from floppy just fine but for some reason the cartridge port doesn't work.
I've heard that some PLA replacements (OTP etc) can have problems with the cart port, but I haven't found that with the Atmel 27C512R at all. I would test connectivity between the EXROM and GAME connections that come from the PLA to the cart slot - make sure those are correct. Maybe you have a damaged trace somewhere around the PLA.
Just for the sake of putting it out there, Commodore *is* known on occasion to have used whatever was left in inventory to make some C64's. Just ask Jan Beta, he'll tell you. :) However, I agree with GadgetUK164... in this case, the first place I would start is to check the EXROM and GAME traces for continuity.
Pretty sure I've worked on more C64's over the years tbh! I repaired tonnes of them when I was in the trade. And whilst you do occasionally see one with an odd RAM chip, it's not that common.
I have the C64 diag+DT and C127 diag+DT, none of them have a ram chip in there, also got a Coprolite diag cart that came with a diag harness i bought separately and there is no ram there either. Just Eproms on all.
The cart I've seen was homebrew I think - I am sure that it was created by a friend of the store owner. It had RAM in there, and the test screen was completely different to any of the ones I've seen.
Plenty of diag carts around, World of Jani lists a few, you can download them and fire them up in VICE to see how they work: blog.worldofjani.com/?p=164
I think were over due a parody video from last time. Instead of "stick it in and out a few times " replace it with "pulsing" lol 🙈🙈🤣 hope your well my friend
Hi any ideas on this one? I had a working 250407 revB board all socketed chips and started to do a re-cap. Replaced C19 O.K. then did C88 and the board is dead. No video output at all not even black screen. When I turn it on I do hear it buzzing but that's it. Could it be the Vic chip has fallen over?
Check the solder points on the caps you've replaced - maybe something is bridged where it shouldnt be. Beyond that it could be that a chip has died from ESD, or something has just come to end of life (maybe PLA or something). Also check your voltages before doing anything else too, make sure 5v and 12v from regulators are correct, check the fuse hasn't blown, also check there's 5v on the main ICs
Would different branded ram chips have different clock timings on them? I would like to assume that its always best to have the same branded ram chips on a machine and not mix and match? Im not 100 on these things but its just what was thinking at the time
I tried the trick with run/stop and the dataset worked. I could hear it loading the game but blank screen. Have ordered a vic 2 chip. Will report back.
So I tried an rf cable directly in to the aerial socket on my tv, tuned it in and to my surprise all works normally. Either there is something wrong with the s-video scart or it just doesn’t play well with the cheap scart to hdmi converter I have with the LGC1. odd as the speccy 128 works perfect with it!
Accidentally used a wrong fuse with my c64 and that coupled with a faulty power supply caused an "out of memory error" and then a black screen. I'm thinking one (or more) ram chips are fried. So bummed out now cause it was working flawlessly and now it doesn't because of one idiot mistake
Putting the wrong sized fuse in shouldn't be a problem - unless something fails and draws too much current, but even then it would be rare for that failed component to kill anything else. That said, I have seen where the CPU fails, pulling the 5v to ~3v and the failed databus then kills the CIA chips too due to the CPU databus failure.
@@GadgetUK164 I replaced two ram chips that were getting very hot. Still won't boot. I'm now beginning to think it's either the PLA or the VIC, but I can't test that cause this is the only c64 I have and after searching on eBay it doesn't seem to make any sense to buy expensive replacement chips if I don't even know if that will get it working again... PS: I probably caused a short circuit while I had the case open with the wrong fuse in now that I think about it
I got a rev A brown C64 with black screen, i measured 5 volts rail (comming from a modern psu) an I got solid 6 ohms impedance (measured on both directions so R impedance) is that ok?
Great stuff. Keep it up! I just wanted to let you know that the J in Jan Beta’s name is pronounced like a Y, just like how you say ya in see ya. It’s not pronounced like the female Jan name, the male version being a Germanic name.
Whoops my bad. I haven’t watched your newer stuff yet as much as the older videos. As a recent subscriber I’d like to get the older things out of the way first and work my way up to the new stuff in case I miss out on some hidden pearls of wisdom.
Been watching your videos and they've been a great help. I've hit a brick wall though. I have 250407 rev b board with a black screen. Dead test said u22 memory was bad, so I soldered an replaced it..still black screen! I'm at a loss. Anyone have any ideas?
@@GadgetUK164 That is next on my list, but fortunately they are not MOS brand and look to be Fairchild's. While I do solder and desolder frequently on modern circuits, I get leary of these 1980's circuits with the sort of tin roll coated traces...damn traces lift if you even look at them. Someone has reworked this board in a couple places and burnt the solder mask off but traces seem to have continuity. The IC's you refer to are memory multiplexers? So would that cause a data line to be held either high or low? I don't have a logic probe but that sounds like a smart thing to test
I think address lines go through those don't they? If you've got a stuck databit, it could be something else entirely, like a bad CIA for instance, although i wouldn't expect it to boot the diagnostic ROM cart if you had a stuck databit all the time.
![PASULOL รามเกียรติ์ ตอนที่ 4 นนทกพบรัก [Ramakien Ep.4: Nonthok in love]](http://i.ytimg.com/vi/tMUQgmZzf_A/mqdefault.jpg)
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GadgetUK164 - Retro Gaming Repairs & Mods Another quality video. Watching now. Still humbled and cannot express my thanks enough. This would of cost me £££ - I'm celebrating by 'eaching' (eating and watching) with some garlic bread and spag-bol 😛😎
You're welcome mate =D LOL @ eaching - I always "each"!
GadgetUK164 - Retro Gaming Repairs & Mods HAHA 😂
Those copper heatsinks are expensive but really good. I use 4 on my VIC and 3 on the PLA typically for my personal breadbins.. And yes, I just started doing repairs back in February starting with my own personal C64 which needed a character rom and 1 bad RAM chip.. I have all the tools all you guys have and have learned so much in such short time so thanks for sharing the knowledge. Its a great hobby, but can get expensive and addictive... Not saying I have a problem, but why do I need 5 C64s! (I just made 4 BWACK c64 savers).
Thanks =D Yes, I love those heatsinks - just waiting on some more to arrive! LOL @ 5 C64s! I have a similar crazy number!!
Excellent analysis. I just now fixed a blackscreen rev c board with your help. Yep, U25 on this one too. thanks!
Great news =D
Really enjoy sitting down with a cuppa and spending the morning watching these repair videos mate. Good job again!
Thanks mate =D
Nice work and thanks for the mention. :) I had very similar confusing results with a broken 7708 and the dead test cartridge a while ago. It seemed to randomly flash differently on each power up. I replaced two RAM chips and broke a trace before I realized it was the multiplexer. ;)
No worries =D Yes, faulty RAM indicated by the Dead Test could be the multiplexers. There's no way to test for that type of problem in software.
I had a similar quest with a 7707 chip in my 1541. The way I finally proved that that chip was faulty was removing it from the board and testing it in my TL866 EPROM programmer which has a very handy test option for a range of 74-series chips. Since the 7707 generic replacement is a 7406 I tested it as that and it told me one of the logic units had failed. The TL866 also tests SDRAMs so quite useful. Drive worked again fine after replacing that chip with a 7406.
Yes - that's ideal if you have a device that can test TTL etc. I've got 5 EPROM programmers and not one of them will test either DRAM or TTL =/
Great video, thank you. FYI, youtube has inserted commercials about every 5-6 minutes.
On mine with a black screen, U 10, 12, 22, 24 all 4 are very hot within 30 seconds. The other 4 are cool. I will be replacing all 8 Hitachi RAM chips and both 257's and hopefully this solves the problem. I will test further with my oscilloscope.
Nice job off course one thing could je share more info on de probe you use, it looks to me a very handy tool
In this video I used a "Logic Probe" - which indicates a high / low / pulsing / high impedance signal, and I also use an Oscilloscope to measure clocks.
Thx I could understand the word probe in de video but not the word logic
Good job on working your way to the solution by testing and solid reasoning skills. One thing I thought of while watching is that if one has a eprom programmer like the TL866 you can test 74xxx series logic chips with it. I have not tried this yet but was about to this weekend on a difficult TRS-80 Model 100. In a case like you had where the symptoms indicated a bad 74xxx part and you don't have one on hand it might be worth hooking up the programmer to test the suspect part.
Thanks! =D Yes, I wish I had an EPROM programmer that could test TTL (I've got 5 different EPROM programmers and not one of them tests TTL lol). I was going to create my own TTL tester using a PIC, I've not got around to it yet but I will do at some point! In the meantime I might buy a TL866!
A good reason to build a fully socketed test board! A lot of people use various games to test the sound playabilit. What do you suggest?
I have a fully socketed board build coming up =D Regards testing the SID, you really need to familarise yourself with how a 6581 and 8580 sound with a number of different games. Then always use those games to test. It's easy to spot when there's a missing channel or bad filter etc. But sometimes problems with ADSR stuff can be subtle.
@@GadgetUK164 great, thanks
Thanks for a great video. It helped a lot with me troubleshooting my black screening C64. Not sure what it was in the end. Measured the fuse, voltages of the voltage regulators which showed very good. Tried removing the SID. Still just black. Then I measured the input voltage on the 74-series chips and it showed only 2.2V... was a bit confused and went back and forth between the regulator and the input and after scratching my head for a moment the input was suddenly showing 5V so I hooked it up and it was running fine. So not really solved as it might return for all I know but this video helped very much in me daring to open up and check around at least =)
Possibly dirty switch or power socket!
I might be misremembering but I recall going back and forth between the output pin of the 7805 voltage regulator showing 4.99V and the input pin on several chips showing 2.2V. Not sure if there could be a failing component somewhere bringing down the voltage or a cracked solder joint somewhere that got better when it heated up. Anyway the video helped a lot and none of the caps looked damaged so not going to repair further that which is no longer broken or it might never work again =)
There are 2 5V rails! The regulator on the C64 motherboard provides 5v ONLY for the VIC and chips next to it. The PSU provides a second 5v rail which feeds most of the other chips on the board. So if you see a low 5v elsewhere on the board (other than the VIC or associated chips in that area), then either your PSU has a low 5v level, or more often than not its the power switch or power socket that is dirty. But you could also have a short somewhere, that would pull the 5v down, but shorts of that magnitude don't generally fix themselves.
I have the same problem with my C64 (Black Screen). These information are precious! Grazie mille
Thanks, no problem! Let me know how you get on with yours!
Yeah i watched that interview of you on Riots channel Gadget..nice to finally see your face..i suggested they interview Jay from Jays vintage Junk sometime too!!!
=D
For schematics, you should look at the "Assy no", not the "Artwork no". If the schematic is for the right assy, they are normally correct, even is the artwork number is different. So in your case your have an assy 250407 and I'm sure the schematic you have matches with that. Even for breadbins, there exist several assy's and their schematics differ (clock circuit and RAM mainly).
Yes, I had forgotten that the clock circuit can vary between boards too - that's a very good point! But in general, assuming you dont have clock problems, you can use pretty much any schematics to check connectivity. The IC numbering can confuse you too I guess if you get schematics for the wrong board.
Nice repair Chris. Anthony over at Riot Retro Gaming will be very pleased, Come to think of it "Big-Balls" and "The Horse" will also.
Thanks =D
I agree with Retro Marky. The best video I've ever seen on a C64 repair explaining the various test procedures when working on a dead board. I've found with the MOS 74LS IC's they are now prone to failure and worth checking / replacing after initial tests (to hopefully help cut repair time). I've also scoped the output pins of some suspect / faulty MOS Multiplexer IC's finding they were still swinging high and low but not sufficent enough to change logic levels. As for Diagnostic cartridge with RAM, even the Micro Sleuth 64 (with seperate LED Userport board ) or the Diagnose 64 (with onboard diag LED's) don't have any RAM. Could the "RAM" have actually been just 74LS logic IC's or ... there were a few programmable battery backed RAM cartriges like the Trigsoft UPC1 could this be what you were thinking of?
Thanks =D Very much appreciated =D I think the cart I used to use at Eteson Electronincs was a homebrew cart - it had that appearance tbh, and I've searched high and low and not found anything that looks like it (either on the board with RAM, on regards how it displayed on screen).
mate this video is SUPER helpful ; i have a dead c64 with white flashing on my diag cart. the mem has been socketed ; my suspicion is the 7708's over the RAM as well but your troubleshooting has me really thinking its them. will socket em up and check. thanks mate
Thanks =D Just be aware that other things can cause black screen (like the PLA, CPU, CIAs, SID etc). But if its not the PLA or RAM then the 7708s are very possible.
yep! i've had the PLA tested in another machine and it works fine. the SID is non existent and the CIAs have been socketed and removed. am told the RAM was replaced (but unsure) so will try some 257s and see how we go.
Fantastic video Mr.G. The best video I've ever seen on a C64 repair. So glad you went into the logic probe. Could you consider making a video on using a scope to diagnose C64 problems? I think a lot of people including myself would learn a lot from that and you're the man to do it ;)
Cheers my friend =D I will do more with the scope, I've got a number of videos to do on some of the basics of electronics and repairs.
Excellent video as always! Lots of red herring opportunites indeed, but you found a solid footing quick by being methodical. Interesting failure mode on those 7708:s
Thanks =D Yes, some red herrings - I hate those lol =D I've still got the faulty chips and will do a follow up video later!
Excellent. Educational. Top notch vid for schooling lesser mortals such as myself. 👍👍
Many thanks =D
Good find. Unfort had to skip thru because my digital motion sickness kicked in, it does when there is lots of close movement to camera.
Sorry about that! I will try to do less swaying and camera movement when up close! =D
There is no need to apologise ;-) Its just your content is so good I want to watch but sometimes when there is too much movement in front of camera the DMS kicks in. I used to think it was just me but its quite common - well.blogs.nytimes.com/2015/11/14/feeling-woozy-it-may-be-cyber-sickness/ for instance
I don't get the problem with say Jay's stuff for instance, he had the camera just that little further back.
Commodore 64 + Hunters Moon is the reason why my usename is Hunters Moon
Hehe =D I guessed that!
This is an incredibly informative video - peerless stuff. Thank you!
Thanks, much appreciated =D
What a pain in the arse. I do enjoy the fault findings but it had me stumped. Glad you got it repaired. Another C64 resurrected...... Great vid Chris 😁😁😁 Kim....
Thanks =D Yes, was a bit of a pain, shame it didn't end after the first 7708 was replaced =D
Thanks for the content. Was debating commenting because this is a couple years old. But this stuff is just too helpful not to say thanks. Great video.
Thanks =D I always reply where I see a message!
It is a good practice to target all MOS logic chips and MT RAM chips. They have a remarkably high failure rate.
I am sure they fail a fair bit, but I've got loads of C64s with those chips on that work fine atm! Ultimately I am sure they will die before other makes perhaps though.
Thanks for this awesome video! I'm working on that exact board. Can't really figure out whats happening. 8500 gets very hot, no phi2 from it. Kernal and CHR is being addressed but not Basic. D4 is stuck, other than that, there is data on the bus. Removed CIA2 and SID since they were socketed. Any further tips on what I should check next? I have an oscilloscope and another C64, but It's a shortboard without any sockets on chips that are compatible.
If there's no PHI2 from the CPU, I would suspect the CPU tbh! I could be wrong, but I think the VIC2 provides the clock to the CPU, and the PHI2 should be created by the CPU.
Yeah, that's what I've gathered too from reading the schematics. I'll order 40 pin sockets and test the dead(?) CPU in my working machine then :)
I have used the Recom 5V regulators in many places, but I wouldn't recommend them for the ones on the C64 as they both drive analogue circuitry and don't use that much power, so would be better off with the original linear regulators.
Yep, that seems to be what Jan found. Spike on the 12v side after power turned off (probably due to capacitors). It did make me wonder if a similar spike was created on the 5v side too though.
Investigation is ongoing. I replaced the 12V Recom one with a regular 7812 for now. I'm using the C64 a lot and haven't found any more issues yet. At least nothing you can tell from the outside except the whole machine is obviously running cooler. I'll watch for spikes on the 5V rail when I open it up again.
Hey great work! That's the first Commodore 6 I've ever seen! :-D
Thanks =D You missed a 4 off - Commodore 64 =D
This was a complicated one. Well done.
Thanks =D It's annoying it didn't end after the first faulty chip was found :P
I have my 5VDC at the regulator, but no 5VDC to the 7400 chips. Any clue?
The power supply itself provides the 5v for the other TTL chips! The regulator only powers the VIC chip and the surrounding chips in the shielded area. Check the 5v on the power connector on the PSU, and check the fuse in the PSU hasn't blown.
I had saw a video, maybe yours, where you measured 5VDC at the 5VDC regulator and then measured 5VDC at the TTL chips.. I assumed that voltage for the TTL's was supplied from that regulator! My mistake.
Ah Ha! Just checked..Less than 2 volts coming off of C91 (and the TTL chips). Maybe something is loading it down(?). Good power supply tested in a working unit. Thanks!
EDIT-------------------------------------------
Just watched the video again and you explained the separate 5 volt sources in the text. I should pay more attention. 3:42 4:10
Feel the tops of the chips - if something is dragging the 5v down to 2v, something will be super hot! My bet is the SID or a CIA.
Always helpful.
Got two I found in the attic...both black screen...would the heat up there cause problems? Gonna do some tip toeing through them
It's unlikely heat in the attic would cause any problems tbh. Let us know how you get on =D
I am really amprehensive about desoldering on these old boards
Provided you don't use too much heat they are OK to work on! Sinclair Spectrums are much more difficult. If you have little desoldering experience its best to get a cheap desoldering station as it makes chip removal much easier. Something like the ZD-915.
I find that if I cut the chips out when I replace them it makes desoldering easier and you don’t spend forever heating the area trying to get chips out.
I wonder why you didn't use your 1541U2 for diagnosing the RAM? It needs just PLA, VIC and somewhat functioning RAM to display a picture on the screen. Inserting it and activating the menu is very usefull for RAM diagnostics: If one line in every character is broken, you have a broken RAM chip. If you see total garbage on the screen, something else is going on.
That's a nice tip! But the main reason I never connect the 1541U2 up whilst fault finding is the risk that a fault could damage the 1541U2. In this case it wasn't likely, but there are some black screen faults where say a data or address bus is stuck high, which can kill other things connected to that bus. I've seen 64C's with a dead CPU, and 2 x dead CIAs - all with the same data bus connection killed. If I connected up a 1541U2 to that, it could damage it! The only time I will use the 1541U2 is when I know its working lol.
Yes, I think some pre-checks are usefull, but I've succesfully used the 1541U2 in 2 black screen repairs until now:
In the first machine, all ROMs were socketed, I removed all of them (even char ROM not needed to 1541U2). After removing all of them, 1541U2 menu worked fine and using the altenate KERNAL function, I was able to get empty blue screen. So RAM was fine, no need to worry about that, and one of the ROMs had to be the cause. It turned out that the BASIC ROM was spoilng the bus, replaced it. C64 working fine.
For the other machine, the 1541U2 menu did show immedeately that one of the RAM chips was defect. Replaced it, machine was working fine for 2 weeks, then broke again :( I'm sure the RAM is correct, I have socketed all RAM chips and tried many things there, but as the 1541U2 menu displays perfectly the RAM is unlikely the cause now. I'm considering desoldering the CPU, but it'll have to wait until I'm in the mood again to look at it.
Great way to test the ROMS there for sure!!! =D
What's your opinion about the heat in the VIC chip having a large mobo like the ones of the breadbin?. Would you recomend to keep the plate over that chip there to dissipate the heat instead of using a heatsink?.
Or thinking about mods, wouldn't it be better to add a fan over that cover instead of using a heatsink with a fan on the VIC?
Or in the case that both solutions could work more or less the same, do you recomend using a fan on the VIC?.
I think it would be better to probably just have a dedicated heatsink and remove the top tbh - but... That top joins the side piece etc and acts as a larger heatsink. So I guess it would be worth testing in both scenarios to see which way the VIC is actually cooler. Personally, after seeing how hot my VIC chip gets I prefer a fan tbh. On the C64C it doesn't get so hot though.
Yes, great video! Thanks Gadget! Now, make one for A500 as well :)
Thanks! Hehe, trying to find faulty Amigas is half of the problem lol.
I wonder if Commodore repaired and repackaged systems, could explain the mismatched chips . Also what do you think could cause a cartridge port from not working besides power or ground issues, I'v replaced my PLA chip with a OTP rom chip after a power supply took it out and the system boots up and loads from floppy just fine but for some reason the cartridge port doesn't work.
I've heard that some PLA replacements (OTP etc) can have problems with the cart port, but I haven't found that with the Atmel 27C512R at all. I would test connectivity between the EXROM and GAME connections that come from the PLA to the cart slot - make sure those are correct. Maybe you have a damaged trace somewhere around the PLA.
Just for the sake of putting it out there, Commodore *is* known on occasion to have used whatever was left in inventory to make some C64's. Just ask Jan Beta, he'll tell you. :)
However, I agree with GadgetUK164... in this case, the first place I would start is to check the EXROM and GAME traces for continuity.
Pretty sure I've worked on more C64's over the years tbh! I repaired tonnes of them when I was in the trade. And whilst you do occasionally see one with an odd RAM chip, it's not that common.
I have the C64 diag+DT and C127 diag+DT, none of them have a ram chip in there, also got a Coprolite diag cart that came with a diag harness i bought separately and there is no ram there either. Just Eproms on all.
The cart I've seen was homebrew I think - I am sure that it was created by a friend of the store owner. It had RAM in there, and the test screen was completely different to any of the ones I've seen.
Plenty of diag carts around, World of Jani lists a few, you can download them and fire them up in VICE to see how they work: blog.worldofjani.com/?p=164
Thanks! I've not found it yet =/
I think were over due a parody video from last time. Instead of "stick it in and out a few times " replace it with "pulsing" lol 🙈🙈🤣 hope your well my friend
🤣
Very nice!
Hi any ideas on this one? I had a working 250407 revB board all socketed chips and started to do a re-cap. Replaced C19 O.K. then did C88 and the board is dead. No video output at all not even black screen. When I turn it on I do hear it buzzing but that's it. Could it be the Vic chip has fallen over?
Check the solder points on the caps you've replaced - maybe something is bridged where it shouldnt be. Beyond that it could be that a chip has died from ESD, or something has just come to end of life (maybe PLA or something). Also check your voltages before doing anything else too, make sure 5v and 12v from regulators are correct, check the fuse hasn't blown, also check there's 5v on the main ICs
Would different branded ram chips have different clock timings on them? I would like to assume that its always best to have the same branded ram chips on a machine and not mix and match?
Im not 100 on these things but its just what was thinking at the time
Yes, and it can make a difference. -20 is 200nS -15 is 150nS -12 120nS etc. But the C64 only needs -20 or faster. So you can mix them without issue.
I tried the trick with run/stop and the dataset worked. I could hear it loading the game but blank screen. Have ordered a vic 2 chip. Will report back.
So I tried an rf cable directly in to the aerial socket on my tv, tuned it in and to my surprise all works normally. Either there is something wrong with the s-video scart or it just doesn’t play well with the cheap scart to hdmi converter I have with the LGC1. odd as the speccy 128 works perfect with it!
Love the video as always
=D
One should allways have desolder braid on standby ☺
=D
Accidentally used a wrong fuse with my c64 and that coupled with a faulty power supply caused an "out of memory error" and then a black screen. I'm thinking one (or more) ram chips are fried. So bummed out now cause it was working flawlessly and now it doesn't because of one idiot mistake
Putting the wrong sized fuse in shouldn't be a problem - unless something fails and draws too much current, but even then it would be rare for that failed component to kill anything else. That said, I have seen where the CPU fails, pulling the 5v to ~3v and the failed databus then kills the CIA chips too due to the CPU databus failure.
@@GadgetUK164 I replaced two ram chips that were getting very hot. Still won't boot. I'm now beginning to think it's either the PLA or the VIC, but I can't test that cause this is the only c64 I have and after searching on eBay it doesn't seem to make any sense to buy expensive replacement chips if I don't even know if that will get it working again... PS: I probably caused a short circuit while I had the case open with the wrong fuse in now that I think about it
Great fix!
Thanks =D
I got a rev A brown C64 with black screen, i measured 5 volts rail (comming from a modern psu) an I got solid 6 ohms impedance (measured on both directions so R impedance) is that ok?
Are you saying you measured between the 5v rail and ground and measure 6 ohms?
Nice repair!
Thanks =D
Good info
Thanks
Great stuff. Keep it up! I just wanted to let you know that the J in Jan Beta’s name is pronounced like a Y, just like how you say ya in see ya. It’s not pronounced like the female Jan name, the male version being a Germanic name.
Thanks, yes - I do say it correctly these days! That was filmed a while back!
Whoops my bad. I haven’t watched your newer stuff yet as much as the older videos. As a recent subscriber I’d like to get the older things out of the way first and work my way up to the new stuff in case I miss out on some hidden pearls of wisdom.
Been watching your videos and they've been a great help. I've hit a brick wall though. I have 250407 rev b board with a black screen. Dead test said u22 memory was bad, so I soldered an replaced it..still black screen! I'm at a loss. Anyone have any ideas?
Did you try swapping the 7708 chips (like in this video)?
@@GadgetUK164 That is next on my list, but fortunately they are not MOS brand and look to be Fairchild's. While I do solder and desolder frequently on modern circuits, I get leary of these 1980's circuits with the sort of tin roll coated traces...damn traces lift if you even look at them. Someone has reworked this board in a couple places and burnt the solder mask off but traces seem to have continuity. The IC's you refer to are memory multiplexers? So would that cause a data line to be held either high or low? I don't have a logic probe but that sounds like a smart thing to test
I think address lines go through those don't they? If you've got a stuck databit, it could be something else entirely, like a bad CIA for instance, although i wouldn't expect it to boot the diagnostic ROM cart if you had a stuck databit all the time.
Why is it snowing inside your house :D
Haha =D It's dust in the air - I really need to get an air purifier in that room I think!